Player numbers are correlated with a game's performance (unless you're dealing with a pay2win mobile game trying to hook in a small nunber of whales, but despite hyperbole, Civ7 isn't that). So I don't buy that argument.
The true doom-level is related to Firaxis' costs which we just don't know, but there'a a lot of mental gymnastics needed to make the player counts into a positive thing.
- Simultaneous player number depends on active player numbers and time they spent in game (which depends on many factors - gameplay, how old the audience is, competition landscape, etc.).
- Active players number is a current number of buyers multiplicated by the current active user percentage (which is quite weird metric actually, because any marketing person would operate with one of 2 types of retention instead).
- Number of current buyer is part of the total buyers, but with only half a year of data, projecting one to the other is very weak.
- Total number of buyers correlates to total money gains through average amount paid by each buyer, including DLCs and expansions. Again, without any expansions released and only first DLC packs, we have nearly zero information.
- Now to estimate the game success we need to compare those money gains to the money spent, another metric we have little knowledge about
So is there any correlation between current number of active players and the potential game success at any moment? Yes it is, but it's so weak that you'll never be able to find it mathematically. And speaking about math, a simple exercise - throughout first half a year of Civ7 we've seen number of simultaneous players changing for more than 10x. Which of the numbers should be used in your hypothetical correlation formula and why?
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But I was speaking in particular about comparing current concurrent player number of CIv7 with the same number of Civ6, which has no sense at all. You may try comparing more or less similar metrics and somehow assume all the outside factors are similar. But here people compare number of simultaneous players for a game which is on the market for 9 year with huge discounts and have much more owners with a game, which is on market for half a year and have much smaller number of owners, but hypothetically could have higher percentage of active players. If you don't know those parameters, those metrics are just incomparable.
P.S. As I said, Civ6 required 2 years to reliably overcome Civ5 in simultaneous number of active players, which means civ games are very slow in losing active players. You could see it on this forum with some people still playing Civ4 or even earlier games. But again, we have no reliable info on these numbers.